Ultrasound Tips: Always, Always, Always ...
/1. ...Scan the bladder as part of a renal ultrasound
When scanning the kidneys, it is important to always scan the bladder as well. As the kidneys and bladder are connected, pathology in one can cause changes in the other. Therefore, scanning only the kidneys puts you at risk of missing relevant pathology in the bladder. Take the images below for example. These images are from an ultrasound of a middle-aged patient who presented to the ED with acute left sided flank pain that woke them from sleep and was associated with nausea. They had an extensive family history of nephrolithiasis, but no personal history. The renal ultrasound demonstrates unilateral left sided hydronephrosis. From this information, one may suspect an obstructing kidney stone. However, as we see below the scan of the bladder demonstrates a large irregular mass in the area of the left ureterovesiculal junction as the cause of the hydronephrosis.
2. …Scan the CONTRALATERAL (if they have one)
In emergency medicine we have a saying, ‘the best EKG is an old EKG’. This is because the old EKG provides us a reference specific to that patient. This can help us discern if any EKG oddities are acutely pathological, or if they are baseline for that patient. The same can be true of ultrasound, but instead of ‘an old one’ we use ‘the contralateral one’. For example, in the images below we are using ultrasound to evaluate retinal artery flow in a patient sent in with acute right sided vision loss. We see no signal in the right eye, but we can appreciate flow in the left eye with the same settings. This suggests to us unilateral pathology rather than a baseline abnormality specific to this patient.
3. .…Try the left lateral decubitus position
The left lateral decubitus position can be helpful for optimizing images in a variety of scans. For biliary and cardiac exams, placing the patient in the left lateral decubitus (LLD) position brings the targeted structures closer to the probe, which improves resolution. Additionally, with biliary scans, LLD positioning may bring an otherwise unseen posterior lying gall bladder into the reaches of the probe (1). The aorta may also be more easily imaged in the LLD position, as it allows you to replace an acoustic window typically composed of bowel and bowel gas, with the more favorable window of the liver (2). This position can similarly optimize imaging of the kidneys. In the LLD position, the liver may serve as an acoustic window for the right kidney. In the right lateral decubitus position the spleen can serve as an acoustic window for the left kidney.
4. …Consider using a Water Bath for Superficial Scans
Soft tissue ultrasound allows for the evaluation of numerous pathologies including foreign bodies, abscesses, distal extremity fractures, and tendon injuries. Unfortunately, performing an ultrasound of small superficial structures can be challenging. Ultrasound may poorly visualize very superficial structures. Additionally, maintaining adequate contact between the patient and the probe can be difficult on uneven surfaces such as fingers or joints. This can be even more challenging when the area involved is tender, or when working with pediatric patients who aren’t cooperative with the exam. While in some cases this can be overcome by applying liberal amounts of gel, an easier alternative may be to use a water bath. This method allows you to utilize the water in place of the ultrasound gel, allowing you to hover above the targeted structure as you scan. Putting a small amount of distance between the target structure and your probe can improve image resolution in very superficial structures, and patients may be more tolerant of the exam as no contact with the probe is required (3).
5. …Use M mode to calculate first trimester heart rate
Ultrasound is known to exert both thermal and mechanical effects on the tissue being viewed. While these effects are insignificant in adult tissues, they have the potential for harm in the delicate first trimester fetus. The relatively loose structure of the early fetus makes it particularly susceptible to ultrasound mechanical shear. Furthermore, an early fetus is also thought to be more susceptible to the thermal effects of ultrasound, because it lacks robust vascular flow, which in adults helps to dissipate the heat transferred by ultrasound. Such detrimental effects are further exacerbated by the fact that early gestations typically fit entirely within the width of the ultrasound beam, resulting in near constant insonation of all structures during an exam. Although no human studies have demonstrated detrimental effects, studies in rats have shown damage to fetal lungs, myocardium, and liver (4,5,6). B mode and M mode are considered safe, and first trimester evaluation in the ED should be limited to those two modes. Evaluation with spectral, color, and power Doppler should be avoided, as the forces exerted by these modes are known to be higher. A fetal heart rate can be calculated by the machine by measuring the distance between the peaks of two beats in M mode, as shown below.
6. …Use longitudinal axis for needle finding
In short axis, only a small cross section of the needle is visible, and it can be difficult to discern whether you are truly appreciating the needle tip, or simply a cross section of the needle shaft. In 2009 a study was completed evaluating the frequency of ‘back walling’ during ultrasound guided vascular access. Twenty-five residents who had previously completed ultrasound guided vascular access procedures, underwent a 2-day training course. They then attempted to place an internal jugular catheter on a phantom in short axis and indicate when they felt the needle was in adequate position. It was found that the posterior wall was punctured at some point during the attempt 64% of the time, and the final location of the needle tip was deep to the posterior wall of the vessel 24% of the time (7). Another study, by Stone et. al, directly compared the frequency of needle visualization at the time of vessel puncture in both longitudinal axis and short axis. They found that the needle tip was visualized at the point of vessel access 62% of the time in longitudinal axis, but only 23% of the time in short axis (8). Give yourself the best chance of putting that needle tip exactly where you want it and give longitudinal axis a try.
7. …Put Color on anechoic Structures before you cut
Many different types of fluid on ultrasound appear entirely anechoic. While some anechoic fluid collections may be infectious, others may be vascular. Therefore, before you put a needle or a knife into anything, you should put color Doppler over the structure. For example, the scan below was performed in a patient presenting with upper extremity redness and pain. On ultrasound we see a large, superficial, well-circumscribed structure that appears to have some swirling in it. Initially you may believe this is an abscess, however when we apply color to it, we appreciate significant flow- this was actually an arterio-venous fistula. Arterial aneurysms can also be misleading on ultrasound. The aneurysm itself may appear as an anechoic fluid structure without flow. However, additional scanning will demonstrate that it connects with an artery with active flow. you already have the ultrasound out, take a few extra seconds to apply color to ensure that the anechoic structure is not contiguous with a vessel.
8. …Look at your landmarks
Sometimes ultrasound can be disorienting, and this can lead to misinterpretation of the images. To help prevent yourself from getting turned around, always look at your landmarks. Here we have an example of subcutaneous emphysema. This finding can make it appear as though lung sliding is absent, when in reality lung sliding is being obscured. An easy way to know that this is subcutaneous emphysema instead of pneumothorax is to look at the ribs. Remember subcutaneous air lies above the ribs, and the pleural line lies below the ribs.
Another example is discriminating between pleural and pericardial effusions. Both pleural and pericardial effusions lie in close proximity to the heart, and it can be easy to mistake one for the other. The key to telling the difference is to find the descending thoracic aorta. Pericardial effusions will be anterior to the descending thoracic aorta. Pleural effusions will be posterior and lateral to the descending thoracic aorta (11).
9. …Optimize your image
When acquiring ultrasound images, it is imperative to ensure your settings are optimized to avoid fallacies in your image. Below is an example of how incorrect settings can lead you astray. In this early pregnancy scan, the initial interpretation was no definite intrauterine pregnancy. However, notice how the near field is dark and it is difficult to appreciate any architecture in that field. Contrarily, the far field is so bright it is almost overstimulating, and again impedes visualization of discrete structures. This is from adjustments in the gain which were already set when the operator started scanning. These adjustments were likely left over from a prior exam-where such adjustments were need- or from accidentally bumping the time gain compensation knobs. When the gain was reset and the image was re-acquired, a definitive intrauterine pregnancy was clearly visualized.
To optimize your image, make sure that you’ve selected the appropriate probe for what you are trying to visualize, taking into account the depth of the structure and anatomical limitations (sometimes you need the phased array to get between those ribs!). Then, choose the correct setting. Most machines will have a set of default settings for each type of scan. These are designed to optimize the visualization of the intended structures and pathologies pertinent to that specific exam. Using an incorrect setting can severely impede your visualization and obscure pathology. Next, adjust your depth to maximize visualization of targeted structures. Finally, look at your gain and time gain compensation, as these are frequently bumped or may not have been reset from the prior exam. For more info on ultrasound settings, take a look at our Knobology post.
10. …Protect your Equipment
The acquisition of accurate ultrasound images requires properly functioning equipment. Unfortunately, even small mishaps can impair the integrity of an ultrasound. A study in 2009 by Mårtensson et. al tested 676 probes across 32 institutions and found that nearly 40% were damaged enough to lead to errors warranting replacement. More than 65% of the errors found were due to delamination and 21% were due to breaks in the cable (10). A follow up study demonstrated similar results (11). Delamination errors are caused by damage to the acoustic lens, matching material or backing material. Delamination is frequently caused by hitting the probe surface, or by drying it with the incorrect disinfectant. Breaks in the cable refers to damage to the cable connecting the probe to the machine, and commonly occurs when loose wires get caught on nearby objects or are run over.
Some damage to probes results in obvious deficits in the image, while other damage can cause more insidious visual impairments.
For example, below are images from a case were a delaminated probe caused a patent ductus arteriosus to be missed with color doppler (10). In this case, there is no obvious visual cue that the image is unreliable, it just appears as though there is no flow when there actually is. When the patient was later re-imaged with a different probe, a widely patent ductus arteriosus was appreciated.
In summary, high quality, accurate images depend on intact probes. Damage to probes is not only costly but can insidiously impact your image acquisition and interpretation. So please, take care of your probes, and be sure to alert your director to any signs/instances of damage. Remember, every time a probe breaks, an ultrasound director’s heart breaks as well. Remember to secure probes in their holders, and cords in their bungees, and only use the designated wipes to clean your probes.
Post by Meaghan Frederick, MD
Dr. Frederick is a PGY-3 in Emergency Medicine at the University of Cincinnati
Faculty Edits by Jessica Baez, MD
Dr. Baez is Ultrasound Fellowship Trained and an Assistant Program Director in Emergency Medicine at the University of Cincinnati.
References
Foster, S. C., & McLaughlin, S. M. (1977).Improvement in the ultrasonic evaluation of the gall bladder by using the left lateral decubitus position. Journal of Clinical Ultrasound, 5(4), 253-256.
Athey, P. A., & Tamez, L. (1979). Lateral decubitus position for demonstration of the aortic bifurcation. Journal of Clinical Ultrasound, 7(2), 154-155.
Blaivas, M., Lyon, M., Brannam, L., Duggal, S., & Sierzenski, P. (2004). Water bath evaluation technique for emergency ultrasound of painful superficial structures. The American journal of emergency medicine, 22(7), 589-593.
Pellicer, B., Herraiz, S., Táboas, E., Felipo, V., Simon, C., & Pellicer, A. (2011). Ultrasound bioeffects in rats:quantification of cellular damage in the fetal liver after pulsed Doppler imaging. Ultrasound in obstetrics & gynecology, 37(6), 643-648.
Jia, H., Duan, Y., Cao, T., Zhao, B., Lv, F., & Yuan, L. (2005). Immediate and Long‐Term Effects of Color Doppler Ultrasound on Myocardial Cell Apoptosis of Fetal Rats. Echocardiography: A Journal of Cardiovascular Ultrasound and Allied Techniques, 22(5), 415-420.
Holland, C. K., Deng, C. X., Apfel, R. E., Alderman, J. L., Fernandez, L. A., & Taylor, K. J. (1996). Direct evidence of cavitation in vivo from diagnostic ultrasound. Ultrasound in medicine & biology, 22(7), 917-925.
Blaivas, M., & Adhikari, S. (2009). An unseen danger: frequency of posterior vessel wall penetration by needles during attempts to place internal jugular vein central catheters using ultrasound guidance. Critical care medicine, 37(8), 2345-2349.
Stone, M. B., Moon, C., Sutijono, D., & Blaivas, M. (2010). Needle tip visualization during ultrasound-guided vascular access: short-axis vs long-axis approach. The American journal of emergency medicine, 28(3), 343-347.
Haaz, W. S., Mintz, G. S., Kotler, M. N., Parry, W., & Segal, B. L. (1980). Two dimensional echocardiographic recognition of the descending thoracic aorta: value in differentiating pericardial from pleural effusions. The American journal of cardiology, 46(5), 739-743.
Mårtensson, M., Olsson, M., Segall, B., Fraser, A.G., Winter, R., & Brodin, L. Å. (2009). High incidence of defective ultrasound transducers in use in routine clinical practice. European Journal of Echocardiography, 10(3), 389-394.
Mårtensson, M., Olsson, M., & Brodin, L. Å. (2010). Ultrasound transducer function: annual testing is not sufficient. European Journal of Echocardiography, 11(9), 801-805.
